Where am I obligated to pay taxes?

You will need to file and pay in both countries. If you are overseas long enough, the US will allow you an exclusion for around $100,000 of earned income. Alternatively, you can take a credit for foreign taxes paid. Effectively, you pay the higher of the two rates.

So long as you are an American citizen you are required to file US tax returns and pay any tax due. (Much of your foreign-earned income will be excusable from taxable income, but only if you file.) If you are resident in Spain, you likely will need to pay Spanish taxes. Probably the two countries have a tax treaty which will enable you to avoid double taxation. Even if not, the US has, and Spain probably has, provisions in their laws designed to avoid double taxation of income.

There are actually several questions here. You must first determine your "tax domicile." Generally speaking, your tax domicile is where you consider your home for a majority of the year. That domicile will be the place of your primary income tax liability. If you live in Spain for all of 2013, for instance, Spain will likely be your tax domicile. You will need to contact a tax adviser, however, to review your situation and give you specific advice on that issue. For as long as you remain a US citizen, moreover, you have an obligation to file a United States tax return. This is true, regardless of where your tax domicile is located. If your tax domicile is outside the US, however, you are likely entitled to a credit for any income taxes you pay to the foreign government, and a large deduction on your income. In many cases, this will mean that you owe nothing to the US government, even though you still have to file a return. You must also pay attention to your self-employment taxes. There may be a treaty between Spain and the US on this issue. A competent tax adviser should be able to research this issue and advise you.

According to the IRS U.S. Citizens and Resident Aliens Abroad If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and paying estimated tax are generally the same whether you are in the United States or abroad. Your worldwide income is subject to U.S. income tax, regardless of where you reside. When to File If you are a U.S. citizen or resident alien residing overseas, or are in the military on duty outside the U.S., on the regular due date of your return, you are allowed an

As a U.S. Citizen, you are obligated to report and pay taxes on worldwide income, regardless of your location. You will be taxed by the U.S. but may receive offsets for foreign taxes paid to other countries.

As long as you remain a U.S. citizen, you are obligated to pay taxes in the United States, no matter where you live and no matter where you earn your income.